DESCRIPTION: (Adapted from applicant's description) The proposed nested case-control study will evaluate the hypothesis that women with elevated levels of high-density lipoprotein (HDL) cholesterol have an increased risk of breast cancer. The source population is a cohort of women members of the Kaiser Permanente Medical Care Program who underwent a routine multiphasic health examination from 1964 through 1972. Among the cohort, more than 2000 cases of breast cancer were diagnosed by 1988.Two hundred breast cancer cases (100 postmenopausal and 100 premenopausal) will be selected for inclusion in this study. For each case, one control matched on age and data of multiphasic exam will be chosen. Exposure status will be levels of HDL-cholesterol measured in stored serum collected at the time of the multiphasic health examination. The primary statistical analysis will evaluate whether baseline prediagnostic HDL-cholesterol levels are higher among women who subsequently developed breast. Other analyses will examine possible differences in pre-and postmenopausal disease, and the relationship of HDL-cholesterol to established risk factors for breast cancer. A positive association between HDL-cholesterol and breast cancer would be of both theoretical and clinical importance. HDL-cholesterol appears to be associated with many established risk factors for breast cancer and this study may provide insight into a common biological pathway for several reproductive and environmental factors which influence the risk of breast cancer. In addition, there are currently no known biochemical screening markers for breast cancer. Measurement of HDL-cholesterol could be an inexpensive, widely available method identifying women who may be at higher risk of breast cancer and would benefit from more intensive surveillance.